The US Coast Guard had this stamp, showing their landing craft and a supply boat, issued in 1945 after WWII had finished.
During the war the US built thousands of cargo ships to help supply Britain. They were of a simple standard design for the US Merchant Marine and were named Liberty Ships.
|USA - 10 October 1945|
|USA - 26 February 1946|
For other examples of watercraft sail to and follow the links left by others.